The present invention relates to a method and an arrangement for continuously measuring the partial pressure of a gas in a sample and, more particularly, to such a method and arrangement which measures the partial pressure of oxygen dissolved in blood flowing through muscle tissue which is covered by a skin layer.
The partial pressure measurement of a gas such as oxygen in a sample of living tissue has recently become of growing medical significance. Recent technical developments have made it feasible to measure the amount of oxygen contained in the blood stream which actually flows to the cells of living tissue, i.e. at the end of the respiration chain, rather than relying on the standard technique of measuring the amount of oxygen which actually enters the lungs, i.e. at the beginning point of the respiration chain.
Thus, it is known to measure the partial pressure of oxygen in blood vessels by using a cathetertype device or by using needle or so-called pin electrodes. Such measuring devices are known in the art and reference may be made to Zeitschrift Fuer Kreislaufforschung, 1971, pages 13- 23, or published German application DAS 1,179,393.
It is also known to measure the partial pressure of oxygen in specific cells at specific locations in living tissue by using micro-needle electrodes (Garching Instrumente Prospekt, Naturwissenschaft 1972, page 544).
Furthermore, it is known to measure the partial pressure of oxygen by using polarographic-type devices which are placed on the outer skin layer of living tissue and are operative to receive and measure the amount of oxygen which diffuses through the skin layer. Such devices are also known in the art and reference may be made in this connection to published German applications DOS 2,145,400 or DOS 2,255,879.
However, these prior-art techniques have not proven altogether satisfactory in obtaining measurement data to directly analyze the status of utilization of oxygen of the body by the oxygen partial pressure of its tissue. Recent experiments (Mikrozirkulation Workshop, April 1974, Volume 5, page 36 ff.) indicate that the partial pressure of oxygen in living tissue is very much lower than in the blood vessels. This shows (publication of Max Planck Gesellschaft, 1974, pages 444-463)that the blood is used by the organism as a very large and very readily available butter supply of oxygen. The relatively large concentration gradient of oxygen between the blood vessels and the surrounding tissue is employed by the living organism for the adaptation of the microcirculation to the oxygen partial pressue as needed in the tissue. The ability to change the microcirculation is different to the various organs of the body. Especially the tissue of the skeleton muscles has a high range of adaptation of the microflow to the need for oxygen of the tissue, and the muscle tissue shows large changes in oxygen partial pressure related to the status of oxygen utilization of the body. Reference can be had to Microvascular Research, Volume 8, 1974/283.
It is desirable to accurately measure the partial pressure of oxygen flowing in the blood stream of skeletal muscle tissue. This is particularly true in case of impending hypoxia, wherein the flow of oxygen quickly varies from moment to moment.